Ideally, a tire is desirable to be a perfect circle, and interior stiffness, dimensions and weight distribution and other features thereof should be uniform around the circumference of the tire. However, the usual tire construction and manufacturing process make it difficult to mass produce such an ideal tire. That is, a certain amount of nonuniformity in the stiffness, dimensions and weight distribution and other features occur in the produced tire. As a result, an undesirable exciting force is produced in the tire while the vehicle is running. The oscillations produced by this exciting force are transmitted to the vehicle chassis and cause a variety of vehicle oscillations and noises including shaking, fluttering, and sounds of the tire vibrations being transmitted inside the vehicle.
Industry standards are available for evaluating nonuniformity of a tire. In one method, a rotating drum, which serves as a substitute for the road surface, presses against a rotatably held tire with a predetermined pressing force (several hundred kilograms), or the tire is pressed against the rotating drum with the predetermined pressing force. The tire and the rotating drum are capable of rotating around their respective rotational axes, in such a way that when either one rotates, the other is also caused to rotate.
In this condition, the tire or the rotating drum is rotatably driven so that the tire rotates at 60 revolutions per minute. As the tire rotates, the exciting force produced by nonuniformity of the tire occurs. This exciting force is measured by one or more force measuring devices (such as a load cell) mounted on a bearing which rotatably supports the tire or the rotating drum, or mounted on a member attached to this bearing. From the measured value, an index that serves to evaluate the nonuniformity of the tire is computed. This measurement is referred to as a uniformity measurement.
Tires on which measurements were performed are classified into those for which the nonuniformity obtained from the index is within tolerable limits and those for which it is not. To the extent possible, tires for which the nonuniformity is outside of the tolerable limits are subjected to processing to decrease the nonuniformity. Tires that have been processed are then subjected to uniformity measurement again; those for which the nonuniformity is within tolerable limits are separated from those for which it is not. Through the procedure described above, only tires judged to have “nonuniformity within tolerable limits” are selected and shipped to customers (or sent to the next step in the tire evaluation procedure).
Although current tire uniformity machines are believed to be effective, it is believed that further improvements can be obtained. Current tire uniformity machines provide test results that are sometimes inconsistent. In determining whether a uniformity machine is reliable, a same tire will be tested five times to ensure that the machine consistently detects and measures any nonuniformities in the tire. An additional sampling of tires are also then subjected to the same uniformity tests. From this collection of test results, various filters can be generated and applied to production tires to filter actual results. As skilled artisans will appreciate, filtering the test results undesirably adds time to the test procedure. Filtering also raises concerns that the filters may be set to exclude tires that are acceptable and, more problematically, tires that are not acceptable may be passed to allowance.
One approach is to generate characterization plots of components of the tire uniformity machine that adversely affect the true uniformity of the tire under test. It has been determined that the forces applied by components of the uniformity machine each have their own unique characteristic that varies from machine to machine. For example, the rotating drum on one uniformity machine has a different characteristic than another rotating drum on a different machine. It is believed that each rotating drum that contacts the tire's surface and each upper and lower spindle and chuck assembly that engages the tire's bead has a unique force characteristic that contributes errors into the uniformity measurements detected by the machine It is also believed that prior attempts to adequately characterize the load wheel and the spindle characterization are deficient. In particular, prior methods did not adequately consider differences between the angular alignment or rotational position of the upper and lower spindle and chuck assemblies. As a result, different angular alignments of the spindle and chuck assemblies result in force contributions to a tire uniformity measurement that are not adequately filtered or that distort the filtered measurements in a way that does not accurately represent a tire uniformity measurement. Therefore, there is a need in the art to generate an accurate characterization of the spindle and chuck assemblies and a need in the art to consistently align the upper and lower chuck assemblies so that the characterization can be consistently applied to tires being tested.